1. Field of the Invention
The present invention relates to a push switch for opening and closing an electronic circuit and an improvement thereof, particularly to a push switch with an improved actuator assembly applied for a keyboard having a light weight and a compact structure.
With advances in information processing apparatus in recent years, a push switch plays a more important role in these apparatus as a communication tool with an operator. The push switches are required to be more compact, lighter in weight and smaller in height, and further to have a more comfortable feel in the depressing operation of the push switch. Due to increased applications in transportable OA (office automation) apparatus, requirements for a more comfortable feeling a low profile and light weight have become stronger.
2. Description of the Related Art
Generally, a push switch is composed of a switching-element assembly which opens and closes an electronic circuit, and an actuator assembly for transmitting a depression action by a finger to the switching-element assembly. As the switching-element assembly of the push switch, many types are known and utilized including a lead switch, mechanical switch, membrane switch, conductive rubber switch, etc., and selectively utilized in accordance with the specific application.
FIGS. 1, 2(a) and 2(b) show an exemplary structure of a switching-element assembly 100 known as a membrane sheet type switch, which is used in a low profile keyboard. FIG. 1 is a perspective view and FIGS. 2(a) and 2(b) are cross sections.
In FIG. 1, the switching-element assembly 100 comprises an upper sheet 111a and a lower sheet 111b of a flexible film of polyester or the like, having respective wiring patterns 113a and 113b and a plurality of contacts 110a and 110b, which are printed thereon using an ink of Ag (silver) or C (carbon). A spacer 112 has holes at the corresponding positions to the contacts 110a and 110b when these sheets are stacked together.
FIGS. 2(a) and 2(b) show two different states of the switching-element assembly 100, in which FIG. 2(a) indicates an off-state of the contacts and FIG. 2(b) indicates an on-state thereof when two contacts 110a and 110b are closed by a depression force onto the push switch.
FIG. 3(a) shows an overall cross section of an exemplary structure of a push switch (also called a push-button switch) of the prior art including a switching-element assembly 100 of the membrane sheet type. The push switch further comprises a support panel 200 of iron or the like, and the switching-element assembly 100 is disposed thereon. A housing 4 is disposed on the switching-element assembly 100, a slider 3 is arranged movable in a hole 40 of the housing 4, and a key-top 2 is fixed on the slider 3. Two springs 70 and 80 are arranged for obtaining a comfortable key-touch feeling when the key-top 2 is depressed by a finger. A key-bottom 5 which is fixed at the end of the spring 80 depresses the switching-element assembly 100 and makes a contact between two contacts 110a and 110b as previously explained. In this type of push switch, all constituent parts disposed on the switching-element assembly play a role of an actuator assembly for the push switch.
FIG. 3(b) shows a cross section of another example of a push switch of the prior art. The difference between the structures of FIG. 3(a) and FIG. 3(b) is that the latter type of the push switch comprises only one spring 80 and an additional elastic member 50 made of a rubber sheet having a spherical shape portion protruding toward the bottom of a slider 3. The elastic member 50 has a protrusion 50a at the center of the inside wall surface, and the protrusion 50a functions as the key-bottom 5 in FIG. 3(a). The push switch of FIG. 3(b) gives a comfortable snap feeling when the contacts are closed. The actuator assembly of FIG. 3(b) is in a broad sense composed of an actuator assembly 50 in a narrow sense and a slider assembly including slider 3, housing 4, key-top 2, spring 80, etc.
Generally speaking, it is known that a key-top stroke length of about 3 to 4 mm is preferable for obtaining the comfortable key-touch feel, and a slider length (length L shown in FIG. 3(a)) of about 12 mm is required in order to obtain a smooth movement of the slider without shake. Even if smaller dimensions are used, an overall height of the push-button switch, which includes support panel 200, switching-element assembly 100, and the actuator assembly such as shown in FIGS. 3(a) and 3(b), requires at least about 10 mm.
In contrast, a switch element having a short stroke length such as about 1 mm to 2 mm has been put into practical application sacrificing the comfortable key-touch feel. However, with regard to the push switches used in an input apparatus which is in frequent use, it is not appropriate to sacrifice the key-touch feel.
Further, in the existing push-button switches such as shown in FIGS. 3(a) and 3(b), most of the constituent parts are made of plastic material such as ABS resin, and the weight of the actuator including the slider assembly comprising housing, slider, key-top, spring, etc. is about 60% of the overall weight of the push switch. A weight increase is partly due to the fact that the stroke length is large in order to obtain the comfortable key-touch feel.
On the other hand, in an application of the push switch into transportable apparatus which requires a low profile and a light weight, the push switch having a stroke length of about 1 mm to 2 mm without spring 70 shown in FIG. 3(a) has been utilized. In this type, there is a problem of contact error, in which a push switch will not make a contact when the key-top is depressed with a light finger touch, because the contacts are closed only when the key-top is depressed to the downward bottom position (collide operation). Therefore, in an application for a keyboard in which a plurality of push switches are used, frequent input errors are experienced and re-input operation is required.
In order to obtain a low profile push switch using switching-element assembly of the membrane sheet type, several types of push switches have been disclosed. Among them, Japanese Unexamined Patent Publications SHO 57-55020 opened Apr. 1, 1982 (same as U.S. Pat. Ser. No. 4,520,248 filed Aug. 15, 1980) discloses that a sheet of elastic foaming material is utilized as an actuator assembly disposed on a membrane type switching-element assembly. Further, SHO 60-127619 opened Jul. 8, 1985 discloses that an actuator assembly composed of a convex-shaped transformable sheet and a planar sheet joined together at the periphery of the above convex-shape, both being of plastic material, are used as an actuator for obtaining a comfortable click feeling. In the above two disclosures, no slider assembly is used for obtaining a low profile of the push switch.